1. Field of the Invention
The present invention relates to a device and method for generating a composite video signal, and more particularly, to a device and method for generating a composite color signal using a digital quadrature amplitude modulation (digital QAM) method, wherein digital color difference signals are modulated by QAM, the modulated digital color signals are combined and converted into an analog composite color signal, and the analog composite color signal is combined with a luminance signal to generate a composite video signal.
2. Description of the Background Art
A conventional video recording/reproducing apparatus converts digital video data into an analog video signal using a digital-to-analog (D/A) converter. The digital video data includes a luminance signal Y and color difference signs Cr and Cb. Cr represents a difference between the red and luminance signals (R-Y), whereas Cb represents a difference between the blue and luminance signals (B-Y). Each of the digital luminance and color difference signals Y, Cr and Cb is separately converted into a corresponding analog signal.
In the conventional video apparatus, the analog color difference signals Cr and Cb are modulated using QAM. The modulated analog color difference signals are combined with each other and then with an analog luminance signal to generate an analog composite video signal. This technique, however, cannot prevent noise from interfering with the generated composite video signal since the analog color signals are modulated. This noise prevents reproduction of a clear analog composite video signal and interferences with a display of high definition video signals, resulting in poor quality video devices.
More particularly, in order to generate and display an analog composite color signal on a video display unit, such as a cathode ray tube (CRT), digital video data including a luminance signal Y and color difference signals Cr and Cb are separately converted into analog signals using D/A converters. To reproduce the digital video data, it is necessary to compose the analog color difference signals Cr and Cb using QAM to satisfy a conventional television standard. Using the analog QAM, the analog color difference signals Cr and Cb are multiplied by a video subcarrier signal fsc (=3.579545 MHz) to establish a 90.degree. phase difference between the two signals. The modulated analog color difference signals are added to each other to generate an analog composite color signal.
FIG. 1A shows such a conventional analog QAM process and FIG. 1B shows a block diagram of a conventional video device performing the analog QAM process depicted in FIG. 1A.
As shown in FIG. 1B. a conventional video device for processing video signals includes a sub-carrier oscillator 1 for generating a signal having a specific subcarrier frequency (fsc), a 90.degree. phase shifter 2, a first equilibrium modulator 3 for modulating a color difference signal Cb according to the fsc signal from the oscillator 1, a second equilibrium modulator 4 for modulating a color difference signal Cr according to a 90.degree.--shifted fsc signal from the shifter 2, and an adder 5 for adding the outputs of first and second equilibrium modulators 3 and 4 to generate an analog composite color signal.
As shown in FIGS. 1A and lB, the analog color difference signal Cb is input to the first equilibrium modulator 3. The oscillator 1 generates a sine signal (sin 2 .pi.fsct) having a period of 1/fsc seconds. The shifter 2 shifts the sine signal by 90.degree. and generates a cosine signal (cos 2 .pi.fsct=sine (2 .pi.fsct+90.degree.)). The generated sine and cosine signals are respectfully input to the first and second equilibrium modulators 3 and 4 for modulating the color difference signals Cb and Cr using analog QAM.
Once the color difference signals Cb and Cb are modulated, the modulated signals are added in the adder 5 to generate an analog composite color signal. This composite color signal is subsequently combined with a luminance video signal (Y) to generate an analog composite video signal.
In the above described conventional video recording/reproducing devices, QAM is performed on analog signals. Therefore, the conventional devices suffer from noise and generate signals of low quality. Furthermore, the components of the conventional devices tend to be temperature sensitive, so that it becomes difficult to generate an accurate composite video signal. Moreover, if the above analog composite video signal with noise were to be recorded and reproduced to a TV monitor, displaying of high definition quality video signals cannot be ensured.